Organosulfones are useful in a number of fields, e.g. the manufacture of plastics (synthetic resins) and fabric finishing, as additives to textile fibers, as dyestuffs or dyeing aids, and as therapeutic compounds in a variety of processes and treatments. Sulfones are particularly convenient surface-active agents in the chemical process arts.
Prior to the system described in the abovementioned copending application, sulfones were made principally by several different techniques whereby an organic compound R--X was reacted with sodium sulfide (Na.sub.2 S) to yield the organic sulfide R--S--R in accordance with the formula: EQU 2 R--X + Na.sub.2 S .fwdarw. R--S--R+2NaX
in this relationship, X is generally a halogen atom, S is sulfur and R is the organic radical.
The organosulfide R--S--R is oxidized in a second stage to the sulfone with an oxidizing medium or by catalytic oxidization in a reaction which can be represented by the formula: EQU R--S--R+O.sub.2.fwdarw. R--SO.sub.2-- R,
where the reaction product is the sulfone.
These processes and others known in the art not only required a plurality of steps, but frequently isolation of intermediates, such as the organosulfide before subsequent steps were undertaken. Such processes are neither economical nor convenient and were time consuming and frequently had poor yields.
These disadvantages were overcome in the system described in the aforementioned copending application which, in turn, is developed further in our publication Elektrosynthese Symmetrischer Und Unsymmetrischer Sulfone, Berichte der Bunsen-Gesellschaft fur Physikalische Chemie (earlier Zeitschrift fur Elektrochemie), Volume 3, Nov. 30, 1973, and our publication entitled Electrosynthesis of Sulfones, Journal of Applied Electrochemistry, Vol. 3 (1973) pages 291to 295.
In our system as described in the aforementioned application, sulfones are produced in a single-stage reaction by electrolyzing sulfur dioxide in an aprotic organic solvent in which the organic compound R--X is soluble and which is provided with a salt for promoting conductivity of the solvent. The electrolysis produces SO.sub.2.sup.- ions which can replace the X groups of the organic compound.
As described in the aforementioned application, the process for the production of sulfones utilizes the fact that the SO.sub.2.sup.- ion can replace certain functional groups of an organic compound in an organic medium (nonaqueous solvent) in which the SO.sub.2.sup.- ion is produced by electrolysis. While the atoms or groups of a number of organic compounds have been found to be replaceable by SO.sub.2.sup.- ions formed by electrolysis of SO.sub.2 in the organic medium, the compounds which are found to be most reactive for this purpose are the organic halogen compounds (i.e. compounds of the formula R--X in which X is chlorine, bromine or iodine), the sulfuric acid esters and the sulfonic acid esters. Thus X may also represent sulfuric acid ester group or the sulfonic acid ester group.
The overall reaction, therefore, can be represented by the formula: EQU 2e.sup.- + 2R--X + SO .sub.2 .fwdarw.R--SO.sub.2 --R + 2X.sup.-
where the electrolysis reaction is represented by the addition of electrons to the reaction system, ultimately resulting in the formation of ions of the replaced functional groups. The product is, of course, the organosulfone R--SO.sub.2 --R.
As noted, the reaction is carried out in an aprotic organic solvent (nonaqueous medium) containing a salt, preferably a quaternary ammonium salt, designed to provide the necessary conductivity for the electrolysis current which transforms the SO.sub.2 into SO.sub.2.sup.-. Preferably the salt is a tetraalkylammonium salt such as tetramethyl or tetraethylammoniumchloride or bromide.
According to the present invention the organic compound is introduced into the medium or constitutes the reaction vehicle in which the sulfur dioxide is dissolved and the system is then subjected to electrolysis. Of course, a system in which the organic compound is in liquid form and can constitute the reaction medium or vehicle as well as one of the reactants, has the advantage that recovery of the sulfone is simplified. Organic compounds which can operate in this manner are dimethylsulfate, chloroacetonitrile and chloroacetone. The latter compounds require no separate solvent.
It has been found to be advantageous to prevent the electrolysis current from exceeding the maximum usable current density that produces only the SO.sub.2.sup.- ions. This can be accomplished by providing in the electrolysis cell a reference electrode which is not traversed by the electrolysis current and controlling the voltage between the reference electrode and the cathode so that with respect to the standard potential of the sulfur dioxide/sulfur dioxide anion REDOX couple (SO.sub.2 /SO.sub.2.sup.-), the potential does not exceed 0.1 volt. It has been found that best results are obtained when the sulfur dioxide concentration in the solution during electrolysis is at least 0.1 mole/liter.
The process of the present invention also has the significant advantage that it is possible to produce polymeric sulfones readily. It is only necessary, to this end, to use an organic compound of the type X--R--X where R is a difunctional organic radical and X is an atom or group replaceable by SO.sub.2.sup.-. The reaction follows the overall formula: EQU 2n(X--R--X) + 2n(SO.sub.2) + 4ne.sup.- .fwdarw. --R--SO.sub.2 --R--SO.sub.2--.sub.n + 4nX.sup.-
where n is an integer, e.sup.- is the electronic charge, R and X have their earlier-stated meanings, and --R--SO.sub.2 --R--SO.sub.2 -- is the repeating group of the polymer.
Of course cyclic sulfones can also be produced from organic compounds having terminal X groups, the C atoms to which they are attached being bridged by the --SO.sub.2 -- group.
It has been found to be most advantageous to carry out the reaction in an electrolysis cell subdivided by a diaphragm or ion-exchange membrane into a cathode compartment and an anode compartment. When an anion ion-exchange membrane is used, the current through the cell is brought about solely by migration of the anions X.sup.- liberated by the cathodic process. The anions traverse the membrane and are oxidized by the anode. When X is a halogen atom, preferably chlorine or bromine, the halogen X.sub.2 is liberated at the anode as the free halogen. The sulfone is formed in the cathode compartment. The system has been found to reduce side reactions which might tend to form impurities. A cell of the character described has been found to have an especially high yield of sulfones.
In the process of the present invention, the sulfones or sulfone mixtures can be separated from the solvent by the distillation or by extraction with the extraction effluent then being distilled. For the extraction solvent, it is preferred to use a compound in which the salts (provided for conductivity) are insoluble. Such a solvent may be chloroform or petrolether. The salt recovered in this manner may be recycled to the cell and even the free halogen may be used in ancillary chemical reactions.